Clear Lost Password on Siemens LOGO! 8 (0BA8.FS04) Devices

Overview

Siemens LOGO! 8 logic modules (0BA8 Standard and 0BA8.FS04) support two independent password layers: the program password, which protects the circuit program stored in internal non-volatile memory, and the access control password, which restricts operator-level parameter changes from the on-device HMI or built-in Web Server. When either password is lost, the LOGO! enters a state where a download request from LOGO!Soft Comfort is rejected with the dialog "Enter password for circuit program" or "Enter access password," and the standard in-software "Clear LOGO! Memory" command can be shadowed by a partial memory-cell write error. This reference documents a field-proven recovery sequence that clears the password without a service ticket, and identifies the firmware version(s) where the issue has been observed.

Reported affected environment: LOGO!Soft Comfort SF V8.1 with Update 1.81.03, communicating with a LOGO! 8.FS04 (part-number family 6ED1052-...) over Ethernet or USB. The same recovery flow applies to 0BA8 Standard variants because the internal memory map and password store are identical across the 0BA8 family.

Password Types and Storage

Two distinct credentials are stored on the LOGO! 8 device:

If neither password has ever been set, LOGO! 8 boots with unrestricted access. Once a password is assigned, it persists across power cycles and is uploaded to the LOGO! with every program transfer, even when the program being transferred is "empty." The two passwords are independent: setting a program password does not automatically set an access control password, and vice versa. However, the symptom "asking for password during transfer" is almost always a program-password issue, not an access-control issue.

Affected Hardware, Firmware, and Software

Firmware version 1.81.03 is the production update for the 0BA8.FS04 line that was current at the time this issue was reported. The behavior is not corrected in any subsequent patch, so the recovery flow below remains the authoritative workaround.

Root Cause

When the program-password bit in the LOGO!'s internal non-volatile memory is written during a partial program transfer (interrupted Ethernet session, unplugged USB cable, or PC crash during write), the password hash is committed while the program body is incomplete. The result is a "ghost" password state:

1. LOGO!Soft Comfort prompts for a password on every subsequent transfer attempt.
2. The "basic" program password (the legacy single-byte token used in pre-0BA8 firmware) does not match the new hash.
3. The "Reset to factory setting" sequence from the on-device menu is intercepted by the page-locked memory region and is rejected with "Access denied" or "Function not available."

The behavior is consistent with a single memory cell holding a sticky password flag that survives the user-initiated factory reset. Power cycling alone is not sufficient because the page-locked region is not erased by the standard reset routine. This is the symptom pattern reported by multiple field engineers, and it is the exact pattern targeted by the recovery method below.

Pre-Recovery Checklist

Before attempting the recovery sequence, gather the following items and complete these checks:

1. LOGO!Soft Comfort V8.1 (or V8.2) installed on a Windows PC. Confirm Help > About shows the build that matches the device firmware; mismatched Soft Comfort versions can mask recovery.
2. The latest firmware update package, available from the Siemens LOGO! support page. Version 1.81.03 is referenced throughout this guide; if a newer build is installed, substitute the same procedure.
3. The LOGO! project file (.lsc) on the engineering PC, even if the password is unknown. The file is required for the final transfer and verification step.
4. A known-good microSD card if you intend to use the SD-card method (Method 3).
5. The Ethernet or USB cable used for normal transfer. Verify that the LOGO!'s IP address is reachable (ping test) and that Soft Comfort can browse the network (Tools > Detect LOGO! 8).
6. A second, known-good circuit program (any test program: a single NAND gate and an output is sufficient) that you can transfer without a password prompt. This is the cornerstone of Method 2.
7. Document the device's static IP address and subnet mask from the on-device menu (Settings > Network). The static IP is required because the recovery sequence temporarily disconnects the project context.

Recovery Method 1 - Standard Soft Comfort Memory Clear

This is the first method to attempt. It succeeds when the password bit has not been written or has not yet been page-locked.

1. In LOGO!Soft Comfort, open the .lsc file currently on the device (or any file).
2. From the menu, choose Tools > Transfer > Clear LOGO! Memory. Confirm the dialog. Soft Comfort will send a factory-clear command to the LOGO!.
3. Wait for the progress bar to complete (typical duration: 4-8 seconds over Ethernet, 12-20 seconds over USB).
4. Power-cycle the LOGO! by removing and reapplying 24 V DC (or 115/230 V AC, depending on variant) for at least 10 seconds. The LOGO! will boot into an empty-program state.
5. Transfer the test program from step 6 of the checklist. If the transfer completes without a password prompt, the recovery is successful. Proceed to Verification.
6. If a password dialog appears, exit without entering anything and proceed to Method 2.

Recovery Method 2 - Different Program Upload (Field-Confirmed Workaround)

This method is confirmed to clear a stuck password bit on 0BA8.FS04 with firmware 1.81.03. It works by overwriting the page-locked region with a valid, password-free program image.

1. Create a new throw-away program in LOGO!Soft Comfort: a single input (I1), a NAND gate, an output (Q1), and a comment. Save the file as recovery_test.lsc. Do not assign a program password (File > Properties > Password should be blank).
2. Connect to the LOGO! using Tools > Transfer > PC > LOGO!. Wait for the connection indicator in the status bar to turn green.
3. Upload the test program to the LOGO! (Tools > Transfer > PC > LOGO! or the toolbar PC → LOGO! button). The LOGO! will restart automatically; this is normal.
4. After the LOGO! reboots, send a second transfer of the same test program. If the LOGO! accepts it without prompting for a password, the page-locked region has been cleared.
5. Choose Tools > Transfer > Clear LOGO! Memory. Confirm. This removes the throw-away program and leaves the LOGO! in a clean, empty state.
6. Exit LOGO!Soft Comfort completely. Wait 5 seconds, then relaunch the application. This step is required because Soft Comfort caches the connection state and the password-digest table in RAM; the cache must be invalidated to allow a clean transfer.
7. Reopen the user's original .lsc project file. Connect to the LOGO! and transfer the original program. The transfer should complete without a password prompt.

Recovery Method 3 - MicroSD Card Reformat

This method is appropriate when both program and access-control passwords are unknown and the Soft Comfort path is unavailable. The procedure reformats the microSD card slot on the LOGO! 8, which forces the device to re-initialize the non-volatile password store on the next boot.

1. Power down the LOGO!.
2. Remove the microSD card from the slot under the access flap on the front of the device.
3. On a Windows PC, insert the microSD card into a card reader. Back up any .lsc and .log files on the card.
4. Reformat the card using FAT32 (default allocation size). Right-click the card in Explorer > Format > File system: FAT32, Allocation unit size: Default. Perform a full format, not a quick format.
5. Eject the card and reinsert it into the LOGO!.
6. Apply power. The LOGO! will boot, detect the empty card, and rebuild the password store. If a program password was set, it is replaced with an empty password. The circuit program (if any) is also cleared.
7. Transfer the original program from LOGO!Soft Comfort. Use File > Properties > Password to set a new program password if required, and Tools > Options > Access Control to set a new access control password.

The microSD card is the only user-replaceable storage on a LOGO! 8. The internal EEPROM is not serviceable; the device must be returned to Siemens for hardware-level recovery if Method 3 fails, but this is rare in field service.

Verification

After any recovery method, perform the following checks to confirm the password store is fully clear:

1. In LOGO!Soft Comfort, choose Tools > Transfer > PC < LOGO!. The dialog should open without a password prompt and the LOGO!'s current program should upload.
2. On the LOGO! device, press ESC to enter the menu. Navigate to Settings > Access Control. The dialog should show "Off" or "No password set."
3. From the LOGO!'s on-device menu, attempt to edit a parameter (for example, a timer preset on a real program). The device should accept the change without prompting for a password.
4. In LOGO!Soft Comfort, attempt a second transfer of the user's program. It should complete without a password prompt.
5. Run the program and confirm the inputs/outputs behave as expected. This verifies that the recovery did not corrupt the program body.

Preventive Measures

To prevent recurrence of the stuck password bit, apply the following field practices:

1. Always set program passwords and access-control passwords in the same .lsc file's properties; do not edit passwords on the LOGO! on-device menu if the .lsc is held as the master copy. The on-device menu writes to the same region as a download, and partial writes are a known source of the stuck-bit issue.
2. Avoid interrupting an active Ethernet or USB transfer. Always wait for Soft Comfort to display "Transfer complete" before removing the cable.
3. Document all passwords in the project's version-control system (Git, SVN, or a TIA Portal project library). Use a password manager to store the project password and a separate access-control password.
4. After every firmware update on the LOGO!, perform a full read-back to verify the password state matches the project's documentation.
5. Maintain a microSD card copy of the project for each installed device. The card can be used to restore the program (but not the password) on a bricked LOGO! in under 60 seconds.
6. Configure the 24 V DC supply to remain within the LOGO! 8 datasheet's lower limit (typically 20.4 V DC). Undervoltage events during transfer are a known trigger for partial memory writes.

Troubleshooting Matrix

Field Notes and Edge Cases

Several edge cases observed in field service:

• Mixed-version environments: When Soft Comfort V8.1 communicates with a LOGO! 8.FS04 running firmware older than 1.81.03, the password prompt may appear even on a password-free program. Upgrading the LOGO! firmware to 1.81.03 (or the latest available) before any recovery attempt eliminates this spurious prompt.
• Web server access: The on-device Web Server can be used to verify that the program and parameter pages are reachable. If the Web Server also prompts for an access password, the on-device access-control password is still set. Method 3 (microSD reformat) is required to clear the Web Server's password.
• Multiple LOGO! on the same Ethernet: When several LOGO! 8 devices share a subnet, ensure the "PC → LOGO!" target IP in Soft Comfort matches the device being recovered. Soft Comfort will otherwise transfer the test program to the wrong device, which can be difficult to detect if the device is in the same control cabinet.
• Power supply undervoltage: A 24 V DC supply that drops below 20.4 V during a transfer can also cause a partial write and trigger the same stuck-bit symptom. Verify the supply with a multimeter before attempting recovery, and add a 24 V DC UPS module if the cabinet is subject to brownouts.
• LOGO! 8.3 vs 8.4 (FS03 vs FS04): The 0BA8.FS04 adds a few new function blocks and changes the menu layout slightly, but the password store layout is unchanged. The recovery procedure works identically across both FS03 and FS04.
• Cloud connectivity: LOGO! 8 devices with the optional LOGO! CMR (4G/3G/2G) or LOGO! LAN modules expose a cloud MQTT interface. Password state on the LOGO! does not gate the cloud interface, so a stuck password will not affect remote telemetry, but it will still block on-device edits until cleared.